The formation of non-covalent directional interactions, such as hydrogen or halogen bonds, is a central concept of materials design, which hinges on using small compact atoms of the 2nd period, notably nitrogen and oxygen, as acceptors. Heavier atoms are much less prominent in that context, and mostly limited to sulfur. Here, we report the experimental observation and theoretical study of halogen bonds to phosphorus, arsenic and antimony in the solid state. Combining 1,3,5-trifluoro-2,4,6-triiodobenzene with triphenylphosphine, -arsine, and -stibine provides cocrystals based on I···P, I···As and I···Sb halogen bonds. The demonstration that increasingly metallic pnictogens form halogen bonds sufficiently strong to enable cocrystal formation is an advance in supramolecular chemistry which opens up opportunities in materials science, as shown by colossal thermal expansion of the cocrystal involving I···Sb halogen bonds.

During the last 50 years a vast number of structural studies on Schiff bases derived from hydroxyaryl aldehydes and their coordination compounds have been undertaken. In this review we present a systematic and brief overview of the most important discoveries and achievements accomplished in this field. The occurrence of Schiff bases (and their complexes) derived from nine most commonly used hydroxyaryl aldehydes in the Cambridge Structural Database (CSD) has been investigated. Keto-enol tautomerism and intramolecular hydrogen bonding in salicylaldimines and naphthaldimines is discussed, with a comment of the most common errors in determining the correct tautomeric form. Also, the interrelationship of crystal packing and the substituents on the Schiff base is studied. Finally, we give a short overview of conformational differences between free Schiff bases and Schiff bases bonded to metal ions as well as some structural characteristics of Schiff base metal complexes.

We present a database and experimental study of three-centered halogen bonds involving bifurcated halogen bond acceptors. Our study encompasses three cocrystals with nitrogen, oxygen and sulfur atoms as acceptors and iodine atoms as donors, as well as an overview of three-centered halogen-bonded systems contained within the Cambridge Structural Database based on organic iodine as the donor atom and nitrogen, oxygen, fluorine, sulfur, chlorine, bromine or iodine atoms as acceptors. The results demonstrate that three-centered halogen bonding arrangements are a possible outcome of cocrystallisation experiments and that they exhibit well-defined geometries. The occurrence of such three-centered interactions is considerably rarer for halogen bonds than for hydrogen bonds and, at the current level of understanding, their formation cannot be readily predicted. The strong preference for binary two-center interactions makes a clear difference between halogen and hydrogen bonds. However, as potential guidelines towards developing halogen-bonded architectures based on bifurcated acceptors, the database search indicates particular preference for bifurcation of neutral sulfur or anionic chloride and bromide acceptors.